Hybrid organic/inorganic materials, in particular siloxane polymers, are excellent candidates for optical materials, in particular for waveguide applications. These hybrid materials share many of the benefits of polymers including rapid material deposition, low processing temperature and amenability to photolithographic waveguide definition, while the silicate backbone increases the hardness and dilutes the hydrocarbon content. This dilution of the hydrocarbon content is important because overtones from C—H vibrations cause optical absorption around the 1.3 and 1.55 μm communications bands.
One potential problem with siloxane polymers is O—H bonds, which also have overtone absorptions around the communications bands and particularly affect the 1.55 μm band. O—H bonds are a particular problem if the siloxane polymers are produced by the known sol-gel process, and the condensation stage is incomplete. In general, the sol-gel process consists of two stages, namely hydrolysis followed by condensation. Water is used to hydrolyse one or more metal alkoxides to produce M—OH groups that condense to form M—O—M linkages, thereby building up a metal oxide network. For example, the liquid methyl triethoxysilane can according to the sol-gel process be hydrolysed:CH3Si(OC2H5)3+3H2O→CH3Si(OH)3+3C2H5OH
And condensed to produce a methyl-substituted silicate:CH3Si(OH)3→CH3SiO3/2+3/2H2O
The CH3-alkyl substituent is unaffected by the hydrolysis and condensation stages. It will be appreciated that as condensation proceeds, the silicate network becomes increasingly entangled, thereby hindering further condensation reactions, resulting in residual SiOH groups that cause absorption. It is also difficult to completely remove the water from the final product, resulting in additional O—H absorption. These problems have resulted in the development of siloxane polymers for optical waveguide applications with various methods for minimising the O—H content. In one example in an aqueous sol-gel system the O—H content is reduced by incorporating a fluorosilane component and using processing methods that encourage condensation. In another example, a non-aqueous method is used to directly condense silanol and alkoxysilane species and since this method does not involve a hydrolysis stage it is not strictly a sol-gel process.